This application claims priority to Japanese Patent Application No. P11-058050, filed Mar. 5, 1999.
The present invention relates to an optical device, for example, suitable for a flat display for extracting light generated in a light emission portion from a substrate to the external, more particularly to an optical device suitable for an organic electroluminescence display using an organic thin film as an electroluminescence layer, and also relates to a base for the optical device.
Recently, interfaces between human being and machines, for example, multi-media oriented articles have become increasingly important. In order for a user to more comfortably, efficiently operate a machine, it is required for the user to nonerroneously, simply, instantly take a sufficient amount of information out of the machine. To meet such a requirement, studies have been made to develop various display devices.
With the miniaturization of machines, display devices concerned therewith have been also increasingly required to be miniaturized and thinned.
For example, the miniaturization of so-called a laptop-type information processing equipment, that is, display device incorporating type information processing equipment, such as a note-type personal computer or a note-type word processor, has significantly advanced, and correspondingly, the technique regarding the liquid crystal displays as the display devices used for the laptop type information processing equipment has been significantly innovated.
Nowadays, liquid crystal displays have been used as interfaces for various articles used in our daily life, for example, needless to say, laptop-type information equipment, small-sized TV sets, watches, and electronic calculators.
The liquid crystal display having a feature of the liquid crystal which is driven at a low voltage, that is, at a low power consumption, has been studied as a primary display device to be used in various interfaces between human being and machines, that is, various display devices ranging from a small-sized display device to a large capacity display device.
The liquid crystal display, however, has no spontaneous luminescence, and therefore, it requires a backlight. The drive of the backlight requires a power larger than a power for driving liquid crystal. As a result, the liquid crystal display has such a limitation in use that the service time is short insofar as the device is driven by a self-contained battery.
The liquid crystal display has another problem that it is not suitable to be used as a large-sized display device because of its narrow viewing angle, and has a further problem that the contrast varies depending on the viewing angle even if the angle is within a specific viewing angle, because of the display mechanism utilizing the orientation state of liquid crystal molecules.
The liquid crystal display also has an inconvenience caused by its drive mode. For example, an active matrix mode as one of the drive modes of the liquid crystal display exhibits a large response speed enough to handle a moving picture, however, since such a mode adopts a TFT drive circuit, it fails to enlarge the screen size because of a pixel failure. The use of the TFT drive circuit is also undesirable in terms of cost reduction.
A simple matrix mode as another drive mode of the liquid crystal display is low in cost and relatively easy in enlargement of the screen size, however, it does not exhibit a response speed enough to handle a moving picture.
On the other hand, studies have been made to develop spontaneous light emission devices such as a plasma display device, an inorganic electroluminescence device, and an organic electroluminescence device.
The plasma display device utilizes plasma emission in a low pressure gas as a display means, and therefore, it is adapted to enlarge the screen size and to increase the capacity, however, it has problems in terms of thinning and cost. Further, the plasma display device is not suitable to be used as a portable device because a high voltage DC bias must be applied for driving the plasma display device.
As the inorganic electroluminescence device, a green color luminescence display has been commercialized, however, since such a device is driven with a DC bias like the plasma display device, it requires a large drive voltage of several hundreds V. Further, it is apparently difficult to develop a full-color inorganic electroluminescence display.
On the other hand, an organic electroluminescence phenomenon has been studied for a long time since a luminescence phenomenon caused by injecting carriers in an anthracene single crystal having a feature of strongly generating fluorescence had been found in the early 1960s, however, on the basis of the facts that the luminance has been low, luminescence of only single color has been found, and luminescence has been generated by injecting carriers to a single crystal, the studies have been continued as basic studies regarding injection of carries in organic materials.
However, in 1987, Mr. Tang and others of Eastman Kodak Company announced an organic thin film electroluminescence device of a laminated structure having an amorphous luminescence layer enabling low voltage drive and luminescence with a high luminance, and since then studies and developments have been extensively made regarding luminescence of three primary colors, R (red), G (green), and B (blue), stability, rise in luminance, laminated structure, production method, etc.
Along with development of new organic materials by molecular design or the like, studies have been extensively made to apply innovative organic electroluminescence display devices having excellent features such as low DC voltage drive, thinner, spontaneous luminescence, to color displays.
The organic electroluminescence device (hereinafter, referred to sometimes as xe2x80x9corganic EL devicexe2x80x9d) has an ideal feature as a spontaneous luminescence type display device, which converts an electric energy caused by injection of a current into an optical energy for exhibiting planar luminescence.
FIG. 13 shows one example of a related art organic EL device 10. The organic EL device 10 is produced by sequentially forming, on a transparent substrate (for example, glass substrate) 6, an ITO (Indium Tin Oxide) transparent electrode 5, a hole transfer layer 4, a luminescence layer 3, an electron transfer layer 2, and a cathode (for example, aluminum electrode) 1 by a vacuum vapor-deposition method.
When a DC voltage 7 is applied between the transparent electrode 4 serving as an anode and the cathode 1, holes as carriers injected from the transparent electrode 5 reach the luminescence layer 3 through the hole transfer layer 4 while electrons injected from the cathode 1 also reach the luminescence layer 3 through the electron transfer layer 2, to cause recombination of the electrons and the holes in the luminescence layer 3. As a result, luminescence 8 having a specific wavelength occurs by the above recombination of the electrons and holes, which is visible from the transparent substrate 6 side.
The luminescence layer 3 may be made from one or more kinds of luminescent materials selected from anthracene, naphthalene, phenanthrene, pyrene, chrysene, perilene, butadiene, coumarin, acridine, and stilbene. A mixture with such a luminescent material may be contained in the electron transfer layer 2.
FIG. 14 shows an another related art organic EL device 10A, in which the luminescence layer 3 is omitted but instead a mixture with the above luminescent material is contained in the electron transfer layer 2, whereby luminescence 8 having a specific wavelength is generated from an interface between the electron transfer layer 2 and the hole transfer layer 4.
FIG. 15 shows an application example of the above organic EL device, in which a laminated body of organic layers (a hole transfer layer 4 and a luminescence layer 3 or an electron transfer layer 2) is disposed between the stripe cathodes 1 and the stripe anodes 5 which cross each other in a form of a matrix, and signal voltages are applied therebetween in a time-series manner by a luminance signal circuit 11 and a shift register incorporating control circuit 12, whereby luminescence occurs at a plurality of crossing points (pixels).
Accordingly, the organic EL device having the above configuration can be used not only as a display but also as a picture reproducing device. In addition, the organic EL device can be configured as a full-color or multicolor organic EL device by arranging the above-described stripe patterns for each of three-primary colors, red (R), green (G), and blue (B).
Where using such an organic EL display device for a display unit composed of a plurality of pixels, the organic thin layers 2, 3 and 4 are held between the transparent electrode 5 and the metal electrode 1, wherein luminescence is extracted on the transparent electrode 5 side. Such a related art organic EL display device has plenty of room for improvement.
In the case of applying a luminescence device to a color display, stable luminescence with a high efficiency is essential, however, even the luminescence device having the optimum structure exhibits a light extraction efficiency of about 20% only, and most of luminescence is dissipated or scattered on the side surface of the base due to total reflection from the interface between the device and the base. As a result, the device is required to be driven at a high voltage for obtaining high enough luminance. Such high voltage drive may cause deterioration of the device.
This means that, even if a luminescence device is improved in luminous efficiency by combination of organic materials newly developed, unless the light extraction efficiency is improved, the stress of the device becomes large to shorten the service life of the device.
An object of the present invention is to provide an optical device having a long service life, which is capable of extracting light generated in a light emission portion, more specifically, luminescence generated in an electroluminescence device to the external through a light transmittable base at a high efficiency, and of obtaining a high luminance even under low voltage drive or low light quantity, and to provide a base used for the optical device.
To achieve the above object, according to a first aspect of the present invention, there is provided an optical device including: a light transmittable base; and a device constituent layer provided on the base; wherein the base includes, at least in a pixel formation region, continuously extending layers arranged to exhibit a cyclic refractive index distribution in the in-plane direction and to exhibit a specific refractive index in the thickness direction; and the base, on the device constituent layer side, has a surface roughness of 300 nm or less.
The optical device of the present invention is configured such that the base includes, at least in a pixel formation region, continuously extending layers arranged to exhibit a cyclic refractive index distribution in the in-plane direction and to exhibit a specific refractive index in the thickness direction, and the base, on the device constituent layer side, has a surface roughness of 300 nm or less, and therefore, the optical device exhibits the following features. The features will be described by example of using an optical guide such as an aggregation of optical fibers as the base. First, in the thickness direction of the base, light generated in the light emission portion is impinged on each continuously extending layer (optical fiber) at an incident angle within a critical angle defining the total reflection, and passes through the continuously extending layer while being efficiently guided in a state being confined in the optical fiber. Next, in the in-plane direction of the base, since the continuously extending layer, or optical fibers are arranged at specific intervals to exhibit the above cyclic refractive index distribution and further the surface of the base is finished into a surface roughness of 300 nm or less, a sufficient and uniform quality of the incident light passes, at least in the pixel formation region, through the aggregation of optical fibers in a state with less irregular reflection or scattering, and is emerged from the aggregation of the optical fibers. As a result, the quantity of the incident light escaped to the side surface of the base is significantly reduced, and therefore, the efficiency of extracting the incident light to the external as transmission light while keeping a sufficient transmission quantity or a sufficient intensity is increased. The optical device of the present invention, therefore, exhibits a high luminance even under low voltage drive or low light quantity.
According to another aspect of the present invention, there is provided a base for an optical device, which is light transmittable and on which an optical device constituent layer is provided, the base including: continuously extending layers arranged to exhibit a cyclic refractive index distribution in the in-plane direction and to exhibit a specific refractive index in the thickness direction; wherein the surface roughness of the base on the device constituent layer side is in a range of 300 nm or less.
According to the base of the present invention, it is possible to allow the above-described optical device provided on the base to exhibit the above-described effects.